/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package io.netty.util.internal;

import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.lang.Thread.UncaughtExceptionHandler;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.security.SecureRandom;
import java.util.Random;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

/**
 * A random number generator isolated to the current thread. Like the global
 * {@link java.util.Random} generator used by the {@link java.lang.Math} class,
 * a {@code ThreadLocalRandom} is initialized with an internally generated seed
 * that may not otherwise be modified. When applicable, use of
 * {@code ThreadLocalRandom} rather than shared {@code Random} objects in
 * concurrent programs will typically encounter much less overhead and
 * contention. Use of {@code ThreadLocalRandom} is particularly appropriate when
 * multiple tasks (for example, each a
 * {@link io.netty.util.internal.chmv8.ForkJoinTask}) use random numbers in
 * parallel in thread pools.
 *
 * <p>
 * Usages of this class should typically be of the form:
 * {@code ThreadLocalRandom.current().nextX(...)} (where {@code X} is
 * {@code Int}, {@code Long}, etc). When all usages are of this form, it is
 * never possible to accidently share a {@code ThreadLocalRandom} across
 * multiple threads.
 *
 * <p>
 * This class also provides additional commonly used bounded random generation
 * methods.
 *
 * //since 1.7 //author Doug Lea
 */
@SuppressWarnings("all")
public final class ThreadLocalRandom extends Random
{

    private static final InternalLogger logger = InternalLoggerFactory
            .getInstance(ThreadLocalRandom.class);

    private static final AtomicLong seedUniquifier = new AtomicLong();

    private static volatile long initialSeedUniquifier;

    private static final Thread seedGeneratorThread;

    private static final BlockingQueue<Long> seedQueue;

    private static final long seedGeneratorStartTime;

    private static volatile long seedGeneratorEndTime;

    static
    {
        initialSeedUniquifier = AccessController
                .doPrivileged(new PrivilegedAction<Long>()
                {
                    @Override
                    public Long run()
                    {
                        return Long.getLong("io.netty.initialSeedUniquifier",
                                0);
                    }
                });

        if (initialSeedUniquifier == 0)
        {
            boolean secureRandom = AccessController
                    .doPrivileged(new PrivilegedAction<Boolean>()
                    {
                        @Override
                        public Boolean run()
                        {
                            return Boolean
                                    .getBoolean("java.util.secureRandomSeed");
                        }
                    });

            if (secureRandom)
            {
                seedQueue = new LinkedBlockingQueue<Long>();
                seedGeneratorStartTime = System.nanoTime();

                // Try to generate a real random number from /dev/random.
                // Get from a different thread to avoid blocking indefinitely on
                // a machine without much entropy.
                seedGeneratorThread = new Thread(
                        "initialSeedUniquifierGenerator")
                {
                    @Override
                    public void run()
                    {
                        final SecureRandom random = new SecureRandom(); // Get
                                                                        // the
                                                                        // real
                                                                        // random
                                                                        // seed
                                                                        // from
                                                                        // /dev/random
                        final byte[] seed = random.generateSeed(8);
                        seedGeneratorEndTime = System.nanoTime();
                        long s = ((long) seed[0] & 0xff) << 56
                                | ((long) seed[1] & 0xff) << 48
                                | ((long) seed[2] & 0xff) << 40
                                | ((long) seed[3] & 0xff) << 32
                                | ((long) seed[4] & 0xff) << 24
                                | ((long) seed[5] & 0xff) << 16
                                | ((long) seed[6] & 0xff) << 8
                                | (long) seed[7] & 0xff;
                        seedQueue.add(s);
                    }
                };
                seedGeneratorThread.setDaemon(true);
                seedGeneratorThread.setUncaughtExceptionHandler(
                        new UncaughtExceptionHandler()
                        {
                            @Override
                            public void uncaughtException(Thread t, Throwable e)
                            {
                                logger.debug(
                                        "An exception has been raised by {}",
                                        t.getName(), e);
                            }
                        });
                seedGeneratorThread.start();
            }
            else
            {
                initialSeedUniquifier = mix64(System.currentTimeMillis())
                        ^ mix64(System.nanoTime());
                seedGeneratorThread = null;
                seedQueue = null;
                seedGeneratorStartTime = 0L;
            }
        }
        else
        {
            seedGeneratorThread = null;
            seedQueue = null;
            seedGeneratorStartTime = 0L;
        }
    }

    public static void setInitialSeedUniquifier(long initialSeedUniquifier)
    {
        ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier;
    }

    public static long getInitialSeedUniquifier()
    {
        // Use the value set via the setter.
        long initialSeedUniquifier = ThreadLocalRandom.initialSeedUniquifier;
        if (initialSeedUniquifier != 0)
        {
            return initialSeedUniquifier;
        }

        synchronized (ThreadLocalRandom.class)
        {
            initialSeedUniquifier = ThreadLocalRandom.initialSeedUniquifier;
            if (initialSeedUniquifier != 0)
            {
                return initialSeedUniquifier;
            }

            // Get the random seed from the generator thread with timeout.
            final long timeoutSeconds = 3;
            final long deadLine = seedGeneratorStartTime
                    + TimeUnit.SECONDS.toNanos(timeoutSeconds);
            boolean interrupted = false;
            for (;;)
            {
                final long waitTime = deadLine - System.nanoTime();
                try
                {
                    final Long seed;
                    if (waitTime <= 0)
                    {
                        seed = seedQueue.poll();
                    }
                    else
                    {
                        seed = seedQueue.poll(waitTime, TimeUnit.NANOSECONDS);
                    }

                    if (seed != null)
                    {
                        initialSeedUniquifier = seed;
                        break;
                    }
                }
                catch (InterruptedException e)
                {
                    interrupted = true;
                    logger.warn(
                            "Failed to generate a seed from SecureRandom due to an InterruptedException.");
                    break;
                }

                if (waitTime <= 0)
                {
                    seedGeneratorThread.interrupt();
                    logger.warn(
                            "Failed to generate a seed from SecureRandom within {} seconds. "
                                    + "Not enough entropy?",
                            timeoutSeconds);
                    break;
                }
            }

            // Just in case the initialSeedUniquifier is zero or some other
            // constant
            initialSeedUniquifier ^= 0x3255ecdc33bae119L; // just a meaningless
                                                          // random number
            initialSeedUniquifier ^= Long.reverse(System.nanoTime());

            ThreadLocalRandom.initialSeedUniquifier = initialSeedUniquifier;

            if (interrupted)
            {
                // Restore the interrupt status because we don't know how
                // to/don't need to handle it here.
                Thread.currentThread().interrupt();

                // Interrupt the generator thread if it's still running,
                // in the hope that the SecureRandom provider raises an
                // exception on interruption.
                seedGeneratorThread.interrupt();
            }

            if (seedGeneratorEndTime == 0)
            {
                seedGeneratorEndTime = System.nanoTime();
            }

            return initialSeedUniquifier;
        }
    }

    private static long newSeed()
    {
        for (;;)
        {
            final long current = seedUniquifier.get();
            final long actualCurrent = current != 0 ? current
                    : getInitialSeedUniquifier();

            // L'Ecuyer, "Tables of Linear Congruential Generators of Different
            // Sizes and Good Lattice Structure", 1999
            final long next = actualCurrent * 181783497276652981L;

            if (seedUniquifier.compareAndSet(current, next))
            {
                if (current == 0 && logger.isDebugEnabled())
                {
                    if (seedGeneratorEndTime != 0)
                    {
                        logger.debug(String.format(
                                "-Dio.netty.initialSeedUniquifier: 0x%016x (took %d ms)",
                                actualCurrent,
                                TimeUnit.NANOSECONDS
                                        .toMillis(seedGeneratorEndTime
                                                - seedGeneratorStartTime)));
                    }
                    else
                    {
                        logger.debug(String.format(
                                "-Dio.netty.initialSeedUniquifier: 0x%016x",
                                actualCurrent));
                    }
                }
                return next ^ System.nanoTime();
            }
        }
    }

    // Borrowed from
    // http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/ThreadLocalRandom.java
    private static long mix64(long z)
    {
        z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;
        z = (z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L;
        return z ^ (z >>> 33);
    }

    // same constants as Random, but must be redeclared because private
    private static final long multiplier = 0x5DEECE66DL;

    private static final long addend = 0xBL;

    private static final long mask = (1L << 48) - 1;

    /**
     * The random seed. We can't use super.seed.
     */
    private long rnd;

    /**
     * Initialization flag to permit calls to setSeed to succeed only while
     * executing the Random constructor. We can't allow others since it would
     * cause setting seed in one part of a program to unintentionally impact
     * other usages by the thread.
     */
    boolean initialized;

    // Padding to help avoid memory contention among seed updates in
    // different TLRs in the common case that they are located near
    // each other.
    private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;

    /**
     * Constructor called only by localRandom.initialValue.
     */
    ThreadLocalRandom()
    {
        super(newSeed());
        initialized = true;
    }

    /**
     * Returns the current thread's {@code ThreadLocalRandom}.
     *
     * @return the current thread's {@code ThreadLocalRandom}
     */
    public static ThreadLocalRandom current()
    {
        return InternalThreadLocalMap.get().random();
    }

    /**
     * Throws {@code UnsupportedOperationException}. Setting seeds in this
     * generator is not supported.
     *
     * @throws UnsupportedOperationException always
     */
    public void setSeed(long seed)
    {
        if (initialized)
        {
            throw new UnsupportedOperationException();
        }
        rnd = (seed ^ multiplier) & mask;
    }

    protected int next(int bits)
    {
        rnd = (rnd * multiplier + addend) & mask;
        return (int) (rnd >>> (48 - bits));
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the given
     * least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @throws IllegalArgumentException if least greater than or equal to bound
     * @return the next value
     */
    public int nextInt(int least, int bound)
    {
        if (least >= bound)
        {
            throw new IllegalArgumentException();
        }
        return nextInt(bound - least) + least;
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between 0 (inclusive)
     * and the specified value (exclusive).
     *
     * @param n the bound on the random number to be returned. Must be positive.
     * @return the next value
     * @throws IllegalArgumentException if n is not positive
     */
    public long nextLong(long n)
    {
        if (n <= 0)
        {
            throw new IllegalArgumentException("n must be positive");
        }

        // Divide n by two until small enough for nextInt. On each
        // iteration (at most 31 of them but usually much less),
        // randomly choose both whether to include high bit in result
        // (offset) and whether to continue with the lower vs upper
        // half (which makes a difference only if odd).
        long offset = 0;
        while (n >= Integer.MAX_VALUE)
        {
            int bits = next(2);
            long half = n >>> 1;
            long nextn = ((bits & 2) == 0) ? half : n - half;
            if ((bits & 1) == 0)
            {
                offset += n - nextn;
            }
            n = nextn;
        }
        return offset + nextInt((int) n);
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the given
     * least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal to bound
     */
    public long nextLong(long least, long bound)
    {
        if (least >= bound)
        {
            throw new IllegalArgumentException();
        }
        return nextLong(bound - least) + least;
    }

    /**
     * Returns a pseudorandom, uniformly distributed {@code double} value
     * between 0 (inclusive) and the specified value (exclusive).
     *
     * @param n the bound on the random number to be returned. Must be positive.
     * @return the next value
     * @throws IllegalArgumentException if n is not positive
     */
    public double nextDouble(double n)
    {
        if (n <= 0)
        {
            throw new IllegalArgumentException("n must be positive");
        }
        return nextDouble() * n;
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the given
     * least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal to bound
     */
    public double nextDouble(double least, double bound)
    {
        if (least >= bound)
        {
            throw new IllegalArgumentException();
        }
        return nextDouble() * (bound - least) + least;
    }

    private static final long serialVersionUID = -5851777807851030925L;
}
